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Physicist, Startup Founder, Blogger, Dad

Sunday, August 19, 2012

Recent human evolution: European height

These results were announced last year at a conference talk, now hot off the press at Nature Genetics. As stated in the abstract below, the results are important because they show that selection pressure can work on existing variation in polygenic quantitative traits such as height (no new mutations required! See also here). Group differences in the phenotype are (according to the analysis) not due to drift or founder effects -- it's selection at work. Of course, this has been demonstrated many times in the lab, but certain people refuse to believe the results could apply to Homo sapiens, over timescales of order 10ky.

Strong signatures of positive selection at newly arising genetic variants are well documented in humans1, 2, 3, 4, 5, 6, 7, 8, but this form of selection may not be widespread in recent human evolution9. Because many human traits are highly polygenic and partly determined by common, ancient genetic variation, an alternative model for rapid genetic adaptation has been proposed: weak selection acting on many pre-existing (standing) genetic variants, or polygenic adaptation10, 11, 12. By studying height, a classic polygenic trait, we demonstrate the first human signature of widespread selection on standing variation. We show that frequencies of alleles associated with increased height, both at known loci and genome wide, are systematically elevated in Northern Europeans compared with Southern Europeans (P < 4.3 × 10−4). This pattern mirrors intra-European height differences and is not confounded by ancestry or other ascertainment biases. The systematic frequency differences are consistent with the presence of widespread weak selection (selection coefficients ~10−3–10−5 per allele) rather than genetic drift alone (P < 10−15).

3. If the results on selection hold up this will be clear evidence for differential selection between groups of a quantitative trait (as opposed to lactose or altitude tolerance, which are controlled by small sets of loci). We may soon be able to conclude that there has been enough evolutionary time for selection to work within European populations on a trait that is controlled by hundreds (probably thousands) of loci.

9 comments:

I've read that Europeans mixing with American Indians have a tendency to produce tall kids because the two groups have different genes promoting height. I've wondered why Mexicans, who have mixed a lot more mixing than us, aren't thought of as being tall, but maybe differences in northern and southern european height gene distribution has something to do with this.

Perhaps you are assuming that Native Americans have the same height. Plains Indians were taller: http://researchnews.osu.edu/archive/tallind.htm , whereas there southern and coastal cousins were shorter.

It sounds like that paper assumes height gene distributions are uniform. "Genetic differences that are important in the heights of individuals approximately cancel in determining the average heights of entire populations." And dose this while trying to contrast their height with the "modern image of American Indians as being sickly victims succumbing to European disease..." presumably accepting these Indian/European genetic differences which are harder to ignore, and brought into sharper contrast by the robust picture he's trying to paint of American Indians. The author also rounds the paper out with the noble savage/ evil white man motif. It's possible the height data point provided could be useful, but I'm a little leery of accepting it from this source.

Nice to show that SNPs for height predict the shift in height from (tall) Northern to (short) Southern Europeans. The paper doesn't really highlight the fact, but drift looks like a real patsy hypothesis for complex traits: In the "no selection, just drift" neutral world, 1,400+ SNPs all have to have happened to "drift" synchonously from North to South... Hence the p value of 0.0000000000000001 against :-) = No drift ever for complex traits

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